Layered mixed-metal phosphonates for high dielectric strength polymer nanocomposites

ABSTRACT

Mixed metal phosphonates are generally provided. The mixed metal phosphonate can generally have the composition: AB(RPO 3 ) 3 , where A is Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Pb 2+ , La 3+ , Co 2+ , Mn 2+ , Fe 2+ , or combinations thereof; B is Ti 4+ , Zr 4+ , Al 3+ , or combinations thereof; and R is an organic group (e.g., aryl group, an alkyl group, an alkenyl group, etc.). The mixed metal phosphonate can be combined with a polymeric material to form a polymeric film. Methods of making the mixed metal phosphonate by combining and reacting a metal oxide and an organophosphonic acid are also provided.

PRIORITY INFORMATION

The present application claims priority to provisional patentapplication Ser. No. 61/207,631 filed on Feb. 13, 2009 titled “LayeredMixed-Metal Phosphonates for High Dielectric Strength PolymerNanocomposites” of zur Loye, et al., the disclosure of which isincorporated by reference herein.

GOVERNMENT SUPPORT CLAUSE

The present invention was developed with government support and fundingfrom the United States Air Force, UTC under award 22040-FA21. Therefore,the government has certain rights in this invention.

BACKGROUND

Commercially available, off-the-shelf capacitors simply cannot meet theextreme requirements of current and future industrial or militaryapplications for intense, transient sources of high voltage pulse power.Furthermore, minimizing the component volume and weight are critical tothe viability of new power storage systems for use in aircraft,spacecraft, and other mobile devices.

Existing dielectric capacitors have quite low energy densities, both onvolume and mass basis. Electrochemical capacitors, including doublelayer capacitors and supercapacitors, offer high energy and powerdensity, but their rate capability is limited by mass transfer andfaradaic reaction rates. No current capacitor technology has thecombination of energy density, power density, and rate capabilityrequired for new systems currently under development or envisioned forthe future.

The maximum volumetric energy density W (J/cm³) stored by a dielectriccapacitor,W=0.5∈₀∈_(r)E² _(bd)   (1)depends on the relative dielectric permittivity (or dielectric constant,∈_(r)) and dielectric breakdown field strength (E_(bd) in V/μm). For aparallel plate capacitor with area A, thickness d, and capacitanceC=∈₀∈_(r)A/d, we have the alternate expressionW′≡AdW=0.5Cd²E² _(bd)=0.5CV² _(bd)   (2)for the maximum energy W(J) stored by a capacitor charged to thebreakdown voltage V_(bd). The most obvious way to increase W′ (or W)would be to choose dielectric materials with the highest possiblebreakdown field strength. Many polymers not only have high values ofE_(bd), but the also offer the additional advantage of processability.Unfortunately, the dielectric constants of most polymers are negligible.

The energy density could also be increased by blending high-∈_(r)inorganic ceramic materials into polymers, leading to higher effectivedielectric constant. Many groups have attempted to disperse commerciallyavailable, high-∈_(r) ceramic oxides, such as barium titanate (BaTiO₃)into polymers followed by fabrication of thin films. Unfortunately, bothexperiment and theory show that the inorganic loading must be quite highto significantly increase the effective dielectric constant. Forexample, the symmetric Bruggeman equation, one of several effectivemedium theories, suggests that increases in ∈_(eff) will not be observeduntil filler loadings are greater than 30% by volume. Conversely, therehave also been many studies that show the Bruggeman equation grosslyoverestimates ∈_(eff) and that achieving high ∈_(eff) values requiresinorganic ceramic particle loadings greater than 50% by volume.

A major problem that develops from high inorganic loadings in polymers(particularly BaTiO₃) is poor dispersion in the polymer matrix. The poordispersion of inorganic in polymer leads to poor ∈_(eff) and poor E_(bd)caused by the domination of the E_(bd) of the defect-rich inorganicfiller network. The recent work by Kim et al. has shown that surfacemodification of BaTiO₃ by various organo-phosphonic acids leads tobetter dispersion of BaTiO₃ particles in the polymer matrix, to a higheffective dielectric constant, and to only about a 50% decrease inbreakdown field strength compared to polymer alone.

The recognition that polymer-filler interfaces can dominate capacitorperformance leads naturally to the concept of polymer nanocompositedielectrics. As such, a need exists for improved dispersion of inorganicloadings in a polymer matrix.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In general, the present disclosure is directed toward mixed metalphosphonates and their methods of manufacture. The mixed metalphosphonate can generally have the composition: AB(RPO₃)₃, where A isMg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, La³⁺, or combinations thereof; B is Ti⁴⁺,Zr⁴⁺, Al³⁺, or combinations thereof; and R is an organic group (e.g.,aryl group, an alkyl group, an alkenyl group, etc.). The mixed metalphosphonate can be combined with a polymeric material to form apolymeric film.

According to one embodiment, the mixed metal phosphonate can be made bycombining a metal oxide and an organophosphonic acid in a sealedcontainer then heating the metal oxide and the organophosphonic acid toa reaction temperature that is above the melting temperature of theorganophosphonic acid and below the decomposition temperature of theorganophosphonic acid. In an alternative embodiment, the mixed metalphosphonate can be made by combining a metal oxide and anorganophosphonic acid in a solvent and boiling the solvent containingthe metal oxide and the organophosphonic acid. In either method, themetal oxide can include ABO₃ where A is Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺,La³⁺, or combinations thereof; and B is Ti⁴⁺, Zr⁴⁺, Al³⁺, orcombinations thereof. The organophosphonic acid can include RPO₃H₂,where R is the organic group.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures, in which:

FIG. 1 shows powder XRD patterns of BaTiO₃ (middle pattern), BaTi(PPA)₃(bottom pattern), and Ba(PPA) (top pattern) according to the Examples.

FIG. 2 shows powder XRD patterns of BaTi(PPA)₃ (middle pattern),SrTi(PPA)₃ (bottom pattern), and SrTiO₃ (top pattern) according to theExamples.

FIG. 3 shows HRTEM imaging of SrTi(PPA)₃ according to the Examples. Thearea circled and labeled 3 represents the EDS spectra taken.

FIG. 4 shows the ³¹P MAS NMR spectra of a) Ti(PPA)₂, b) Ba(PPA), and c)BaTi(PPA)₃ according to the Examples.

FIG. 5 shows the ³¹P MAS NMR spectra of a) BaTi(PPA)₃, b) physical mix,and c) solvothermal mix according to the Examples.

FIG. 6 shows the ³¹P MAS NMR spectra of mixed-metal phosphonates a)BaTi(PPA)₃ and b) SrTi(PPA)₃ according to the Examples.

FIG. 7 shows the thermal gravimetric curves of BaTi(PPA)₃ (labeled “A”)and SrTi(PPA)₃ (labeled “B”), according to the Examples.

FIG. 8 shows the dielectric constant values of polystyrene composites asa function of weight loading, where A is BaTiO₃, B is BaTi(PPA)₃, C isSrTiO₃, and D is SrTi(PPA)₃, according to the Examples.

FIG. 9 shows the IR spectra of BaTi(PPA)₃ (top plot) and SrTi(PPA)₃(bottom plot).

FIG. 10 shows that heating the BaTi(PPA)₃ eliminated the coordinatedwater starting at 400° C. as evidenced by the TGA curve. BaTi(PPA)₃.H₂Ois labeled “A”, while the anhydrous BaTi(PPA)₃ is labeled “B”.

FIG. 11 shows the IR spectra showing loss of coordinated water in theregion 2000-2300 cm⁻¹, according to the Examples. The top spectrumcorresponds to BaTi(PPA)₃ heated to 400° C., and the bottom spectrumcorresponds to BaTi(PPA)₃ with no heating.

FIG. 12 shows the HRTEM images along with the corresponding sectionsused for the EDS analysis of a single stack of the mixed-metalphosphonate made according to the Examples.

FIG. 13 shows the powder X-ray diffraction data collected on Ti(PPA)₂,made according to the Examples.

FIG. 14 shows the powder X-ray diffraction data collected onsolvothermal mix, made according to the Examples.

FIG. 15 shows the powder X-ray diffraction pattern of the physicalmixture of Ba(PPA) and Ti(PPA)₂.

FIG. 16 shows the EDS results for SrTi(PPA)₃ platelets imaged by HRTEM(FIG. 3), and XPS results for BaTi(PPA)₃ and SrTi(PPA)₃ bulk powders,according to the Examples.

DETAILED DESCRIPTION

Reference now will be made to the embodiments of the invention, one ormore examples of which are set forth below. Each example is provided byway of an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly, and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

Generally speaking, the present disclosure is directed to mixed metalphosphonates and their methods of making. The mixed metal phosphonatescan be synthesized from a metal oxide(s) in combination withorganophosphonic acids. As such, the mixed metal phosphonates includethe organic phosphonate within their chemical structure, as opposed toorganically coated metal oxide particles. Thus, the mixed metalphosphonates contain an organic phosphonate group within the compoundstructure (e.g., covalently bonded), and can provide compatibility tothe mixed metal phosphonates with other organic materials (e.g.,polymeric material). For example, the organic group can be selected toprovide compatibility (e.g., solubility, dispersability, etc.) withparticular polymeric material and/or solvents. Additionally, the mixedmetal phosphonates can still exhibit dielectric behavior similar to, butnot tied to, their starting metal oxide. Thus, the mixed metalphosphonates are a new class of dielectric materials and, therefore, canbe used to form polymeric dielectric films.

The metal oxide(s) used to synthesize the mixed metal phosphonate cangenerally be represented as ABO₃, where A is Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺,Pb²⁺, La³⁺, or combinations thereof and B is Ti⁴⁺, Zr⁴⁺, Al³⁺, orcombinations thereof. In one particular embodiment, the A and B cationscan be paired such that A includes a cation having a valence of +2 and Bincludes a cation having a valence of +4. Alternatively, the A and Bcations can be paired such that A includes a cation having a valence of+3 and B includes a cation having a valence of +3. Examples of suitablemetal oxides include, but are not limited to, BaTiO₃, SrTiO₃, LaAlO₃,BaZrO₃, PbTiO₃, and mixtures thereof.

The metal oxide can be reacted with an organophosphonic acid, generallyrepresented as RPO₃H₂, where R is the organic group. The organic groupcan be any suitable organic substituent. For example, suitable organicgroup include, but are not limited to, alkyl groups (e.g., methyl,ethyl, propyl, butyl, etc.), alkenyl groups (ethylene, propylene,butylene, etc.), aryl groups (e.g., phenyl, benzyl, etc.), andcombinations thereof Additionally, organophosphonic acids containingheteroatoms, such as sulfur, chlorine, bromine, nitrogen, etc. in theorganic group can be used.

Generally, the mixed metal phosphonate formulated from the metaloxide(s) and the organophosphonic acid(s) can be represented asAB(RPO₃)₃, where A, B, and R are as discussed above.

In one particular embodiment, the organic group can be a phenyl group(where the organophosphonic acid is phenylphosphonic acid, known as“PPA”) to form a mixed metal phosphonate of the formula AB(PPA)₃, whichcan readily disperse into toluene allowing easy fabrication ofpolystyrene-mixed-metal phosphonate nanocomposites.

The mixed metal phosphonates can be formed according to any suitablemethod. Suitable methods can include solution reaction methods, meltreaction methods, etc. The method of formulation can be selectedaccording to the particular metal oxide(s) and organophosphonic acid(s)to be reacted.

For instance, the solution reaction method can be used to form the mixedmetal phosphonates without limitations on the type of organic group inthe organophosphonic acid, as long as both the metal oxide(s) and theorganophosphonic acid(s) can be combined into a common solvent. However,since there are numerous available solvents, one skilled in the artshould be able to find a common solvent for nearly every R groups on theorganophosphonic acid(s). According to this method, the metal oxide(s)and the organophosphonic acid(s) are combined into a solvent(s) andheated under solvothermal conditions (e.g., heated above the boilingpoint of the solvent or solvent combination). As the mixed metalphosphonates begin to form, the mixed metal phosphonates willprecipitate out of the reaction solution, and can be collected. Thesolvent can, in one embodiment, be selected to deprotonate the twohydroxyls of the phosphoric acid to facilitate the formation of themixed metal phosphonates. Suitable solvents can include, but are notlimited to, alcohols (e.g., methanol, ethanol, propanol, butanol, etc.),toluene, dimethyl sulfoxide (DMSO), hexane, benzene, chloroform, diethylether, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide,acetic acid, formic acid, water, etc., and mixtures and combinationsthereof.

In one embodiment, a stoichiometric excess of the organophosphonic acidcan be included in the solution when compared to the stoichiometricamount of metal oxide present. Thus, the reaction can be conducted untilall of the metal oxide is reacted into a mixed metal phosphonate. Then,the excess organophosphonic acid can be washed from the precipitatedproduct.

The melt method is generally a condensation reaction that involvescombining the metal oxide(s) and the organophosphonic acid(s) in a tube,sealing the tube to prevent the escape of gas (particularly water vapor,as water is a by-product of the condensation reaction, but is believedto be necessary for coordination to the metals and thus allowing thebridging phosphorus to be present) and heating the organophosphonicacid(s) above its melting point. Upon melting the organophosphonicacid(s) can react with the metal oxide(s) to form the mixed metalphosphonates. In particular, using a stoichiometric mixture (or a slightstoichiometric excess of organophosphonic acid, the acid meltscontaining the oxide, and as the product forms it becomes a solid. Whenthe reaction is finished, only the mixed metal phosphonate product isleft, with the slight excess of acid (if used) that can be washed out.Thus, similar to the solution method, the mixed metal phosphonate willprecipitate out of the solution as it is formed. This reaction can beallowed to continue as desired, for example about 1 hour to about 1week. In most situations, the reaction time can be 12 hours to 60 hours,such as 24 hours to 48 hours.

The melt method can provide more uniform reaction properties acrossdifferent metal oxides. However, the melt method is limited to thoseorganophosphonic acid(s) having a melting point lower then itsdecomposition point. Thus, not all R groups on the organophosphonicacid(s) can be utilized in the melt method.

The products formed according to both of these methods are crystalline.

No matter the method of formulation, the mixed metal phosphonate can becombined with an organic polymeric material. For instance, the polymericmaterial can be solubilized in a solvent. The mixed metal phosphonatecan be dispersed into the solubilized polymeric material to form asubstantially homogeneous mixture. The mixture can then be formed into afilm by any method (e.g., spun coated, film cast, and the like).

The polymeric material can be selected to be compatible with theparticular R group in the mixed metal phosphonate, so that the polymericmaterial and the mixed metal phosphonate can be more readily combined.For example, when the R group is a phenyl group, polystyrene can be usedas the polymeric material and toluene can be used as the solvent.

EXAMPLES

Mixed-metal phenyl phosphonates have been successfully synthesizedstarting from the dielectric oxides BaTiO₃ and SrTiO₃. According topowder XRD patterns, the compounds are isostructural to their respectivedivalent metal phosphonate. From the HRTEM images of isolated stacks ofplatelets coupled with EDS, the distribution of the metals (Sr:Ti) wasseen to be 1:1. From ³¹P-NMR experiments conducted on the mixed metalphosphonate system, that if the two metals separate from [BaTi]⁶⁺ toBa²⁺ and Ti⁴⁺, the Ti-phenyl phosphonate is the major product at 90%.The EDS results confirm that the metals stay together. Thermal analysisof the compounds further confirms the formation of the mixed metalsystem, since from powder XRD analysis of the thermal degradationproducts revealed a mixed metal pyro-phosphate, ATi(P₂O₇)_(1.5), A=Ba,Sr.

Most importantly, these compounds still exhibit the same dielectricbehavior as their starting oxide. The phenyl groups protruding in theinter-layer, allow for better dispersion of the compound into toluene.The dielectric permittivity of the PS composites increases with thebetter dispersion of the dielectric material. This is clearly seen inthe direct comparison of the unmodified BaTiO₃ versus BaTi(PPA). Thenext step in the dielectric characterization of these mixed-metalphosphonates is to measure their effect on the breakdown voltage.Preliminary results on the breakdown voltage show no decrease in thevalue.

Experimental

Materials

Chemicals were obtained as reagent grade from commercial sources andused without any further purification.

Structure Characterization

FT-IR spectra were recorded on a Perkin Elmer Spectrum 100 using ATRdiamond cell method. Powder X-ray diffraction patterns were collectedusing a Rigaku DMAX 2200 diffractometer. Thermogravimetric analysis wasperformed using a Thermal Analysis (TA) SDT-Q600 simultaneous DTA/TGAsystem in an oxidative environment. The samples were heated to 800° C.at a heating rate of 10° C./min. Solid-state 31P spectra were collectedon a Varian Inova 500 spectrometer operating at 202.489 MHz using a DotyScientific 4 mm/XC magic angle spinning (MAS) probe. Bloch decays of 50msec were collected with a 200 ppm window after 45 degree excitationpulses. A relaxation delay of 10 secs was used between each transient.TPPM dipolar decoupling with a field strength of 45 kHz was appliedduring acquisition. A MAS speed of 10 kHz was used, and between 16 to 64scans were collected for each run. Spectral deconvolution was performedwith the standard routine included with Varian's VNMR 6.1 C software.Platelet imaging and elemental analysis was collected using a JEOL 2100F200 kV field-emission gun TEM/STEM with an Oxford Instruments INCA EDSsolid-state X-ray detector. The capacitance measurements at 100 kHz and1 MHz were made using a Keithley 590 CV Analyzer instrument. Impedancemeasurements from 10 Hz to 100 kHz were made using a 273A ModelPotentiostat and EG&G 1025 Model Frequency Response Detector.

Hydrothermal Synthesis of AB(RPO₃)₃ [AB(RPA)]

AB(RPA) were synthesized by the hydrothermal method. 1 mmol of ABO₃ wasadded to a 0.124 M solution of organophosphonic acid, RPO₃H₂, and thereaction mixture was sealed in a Teflon-lined steel bomb. The bomb washeated to 150° C. for 72 h. The powder products were filtered via vacuumfiltration and washed with ethanol to remove unreacted reagents. Thesamples were dried in a convection oven overnight at 80° C.

Melt Synthesis of AB(RPO₃)₃[AB(RPA)]

AB(RPA)₃ were synthesized by a melt reaction involving stoichiometricamounts of ABO₃ (1 mmol) and 10% molar excess of organophosphonic acid(3.1 mmol). The starting reagents were thoroughly mixed in a mortar andpestle for thirty minutes, before being transferred to sealed glasstubes. The reagents were heated to slightly above the melting point ofthe organophosphonic acid for 12 hours. The resulting product was thenwashed with ethanol while under sonication to remove excessorganophosphonic acid. After washing with ethanol, the products werecentrifuged and the supernatant was removed. The resulting product wasthen allowed to dry in a convection oven at 80° C. overnight.

PS Film Preparation for Dielectric Measurements

Polystyrene (PS, average MW ˜100,000) was dissolved in 20 mL of toluenein a glass beaker under rigorous magnetic stirring for 4 hours.Composite materials (ABO₃ and AB(RPA)) were added such that the weightpercentage of the inorganic material was 40 and stirred for 15 hours toobtain a slurry. The viscosity of the slurry was adjusted by addingtoluene. A thin aluminum foil (average thickness 15 μm) was carefullywrapped around a suitable substrate (silicon or Pyrex discs), whichserved as the bottom electrode. Air was blown on the surface for 10minutes and was cleaned thoroughly with ethanol. Next, the PS-compositeslurry was spin-coated on these substrates at 1000 rpm for 15 seconds.The spin-coated substrate was then placed in an oven at 80° C. undervacuum for 12 hours. Finally, parallel plate capacitors were formed bysputter depositing aluminum or gold as top electrodes on the filmsurface through a shadow mask. The results show no significant variationbetween Al—Al and Au—Al capacitors.

Results and Discussion

Powder X-ray Diffraction

Phase purity of the final products, BaTi(PPA)₃ and SrTi(PPA)₃ werechecked by powder X-ray diffraction. FIGS. 1 and 2 show the diffractionpatterns of the starting materials and the products. The diffractionpatterns of BaTi(PPA)₃ and SrTi(PPA)₃ consist of several evenly spacedlow-angle peaks that are characteristic of 001 reflections of layeredsolids. The diffraction lines for BaTi(PPA)₃ and SrTi (PPA)₃ overlayalmost perfectly, indicating that both the compounds are structurallysimilar. This similarity is to be expected given that both compoundscontain the same pendant phenyl group. Furthermore, the slight shift ofthe BaTi(PPA)₃ pattern toward lower angles is consistent with the largersize of barium relative to strontium. Attempts to index the powder X-raydiffraction patterns resulted in an interlayer separation of 15.754(1) Åand 15.351(1) for BaTi(PPA)₃ and SrTi(PPA)₃, respectively. Due topreferred orientation, only the first three diffraction lines could beused to index the patterns. The diffraction pattern for the single metalphosphonate, Ti(PPA)₂ (see supporting information), is featureless,indicating an amorphous structure³⁰. Data from NMR, TGA, and XRD ofthermal degradation products (vide infra) support the assertion that thereaction product is a mixed metal phosphonate, BaTi(PPA)₃, rather than amixture of layered Ba(PPA) and amorphous Ti(PPA)₂.

³¹P MAS NMR Experiments

³¹P MAS NMR experiments were carried out to investigate the phosphorousenvironments in BaTi(PPA)₃, in the physical 1:1 mixture of Ba(PPA) andTi(PPA)₂, and in the solvothermal Ba+Ti+PPA product. ³¹P MAS NMR spectrafor the pure metal phosphonates, Ti(PPA)₂ and Ba(PPA) were alsocollected to compare them with the phosphorous environments in the mixedmetal samples. The presence of peaks in the ³¹P MAS NMR spectra providesa qualitative indication of different phosphorous environments in thevarious samples.

The phosphorous environments differ in Ti(PPA)₂ vs. Ba(PPA). In theTi(PPA)₂ structure, the octahedral coordination sphere around each Ti⁺⁴cation consists of six oxygens from six different phosphonate groups;each phosphonate group bridges three Ti⁺⁴ cations. A consequence of thisstructural arrangement is that the TiO₆ octahedra are isolated from eachother. By comparison, in the Ba(PPA) structure, the coordinationenvironment around each Ba⁺² cation consists of six oxygens, two ofwhich belong to a single phosphonate group, three of which belong tothree other different phosphonate groups, and one that is part of awater molecule; each phosphonate group bridges three Ba⁺² cations. Aconsequence of this structural arrangement is that each BaO₆ octahedronis connected via a shared oxygen to four other BaO₆ octahedra. In bothTi(PPA)₂ and Ba(PPA), the crystal structure suggests that thephosphorous will have a single coordination environment in each of thesematerials. This is confirmed by the ³¹P MAS NMR spectra. The ³¹Presonances in Ti(PPA)₂ and Ba(PPA), as shown in FIGS. 4 a and 4 b, occurat −3.92 ppm and 9.25 ppm respectively, indicating that in eachstructure there is only a single, unique phosphorous environment. It isimportant to point out that the chemical shift of 12.40 ppm, observed inthe protonated Ba(C₆H₅PO₃H)₂, is not seen, and that only the chemicalshift of 9.25 ppm, associated with the deprotonated BaC₆H₅PO₃, is seen.

In contrast, the ³¹P MAS NMR experiments indicate that in BaTi(PPA)₃(FIG. 4 c), three distinct phosphorous environments are present. Thefirst phosphorous peak at −3.95 ppm (s) corresponds to the M⁴⁺—O—Penvironment found in Ti(PPA)₂ as seen in the standard ³¹P MAS-NMRspectrum of pure Ti(PPA)₂. Another phosphorous resonance is observed at9.25 ppm and corresponds to the M²⁺—O—P environment in Ba—O—P as seen inthe standard ³¹P MAS NMR spectrum of pure Ba(PPA). An integration of thearea under the peaks indicates that the resonances correspond to ⅓Ba—O—P to ⅔ Ti—O—P. This is in good agreement with the expectedcomposition and structure of BaTi(PPA)₃, where charge balance woulddictate that one PPA group is associated with each divalent barium andtwo PPA groups are associated with each tetravalent titanium. A thirdphosphorous resonance is observed at 3.38 ppm in the BaTi(PPA)₃material. This is a new resonance and has no counterpart in either ofthe single metal phosphonates. The third phosphorous environment,located between the other two phosphorous resonances, is unique to theBaTi(PPA)₃ structure and suggests a phosphorous environment influencedby both the titanium and the barium. We believe that this environmentcorresponds to the phosphonate bridging motif Ba—O—P—O—Ti.

Consistent with this assignment of the resonance at 3.38 ppm is theobservation that this resonance is not observed in simple physicalmixtures of the two individual metal phosphonates. In the ³¹P spectrumfor the physical mixture (FIG. 5 b), we do not observe the 3.38 ppmresonance, although we do see an additional resonance at 12.40 ppm³³,due to the presence of Ba(PPAH)₂, an impurity that forms during theBa(PPA) synthesis. In the ³¹P spectrum for the solvothermal mixture,FIG. 5 c, the 3.38 ppm resonance is again not observed. However thespectrum does show a strong peak associated with the kinetically favoredproduct, Ti(PPA)₂ as well as a weak resonance due to the presence of asmall amount of Ba(PPA).

Analogous results are obtained for SrTi(PPA)₃. FIG. 6 compares the ³¹PMAS NMR spectra for SrTi(PPA)₃ and BaTi(PPA)₃. The ³¹P MAS NMR spectrafor SrTi(PPA)₃ shows three phosphorous environments like BaTi(PPA)₃. Thefirst phosphorous environment located at 10.08 ppm relates to the Sr—O—Pand the Ti—O—P environment is located at −4.31 ppm. Similar to theBaTi(PPA)₃ material, a third phosphorous environment is seen between theSr—O—P and Ti—O—P environments at 3.15 ppm. We believe that thisphosphorous environment corresponds to the Sr—O—P—O—Ti bridging motif.

Infrared Spectroscopy

FTIR spectra (FIG. 3) provided the insight into the binding of thephenyl phosphonic acid to the mixed metal oxides. Binding is evidencedthrough the strong C—H phenyl stretching frequency at 3053 cm⁻¹. The C═Caromatic stretches are seen at 1590, 1484, and 1436 cm⁻¹. Two P—Ostretches are seen: M²⁺—O—P at 1213 cm⁻¹ and M⁴⁺—O—P at 1148 cm⁻¹.Finally the out-of-plane bending of the mono-substituted phenyl ring isseen at 746, 718, and 685 cm⁻¹.

Thermal Analysis.

To determine the thermal stability and degradation products of the mixedmetal phosphonates, thermal gravimetric experiments coupled with powderXRD were carried out. The thermal degradation process of the mixed metalphenyl phosphonates is seen in FIG. 7. Powder XRD analyses of thethermal degradation products confirm the formation of MTi(P₂O₇)_(1.5).Our observed weight loss corresponds to the loss of three phenyl groups.Both mixed metal phenyl phosphonates show incredible thermal stabilitynear 600° C. as seen in the thermal gravimetric curves of the mixedmetal phosphonates seen in FIG. 12.

Platelet Elemental Analysis with XPS and HRTEM/EDS

To analyze the chemical composition of the mixed-metal phosphonates andto confirm the layered structure of these materials high-resolutiontransmission electron microscopy (HRTEM) images were collected andenergy dispersive spectroscopy (EDS) data for the samples were obtained.EDS was used to determine the metal composition of the mixed metalphosphonates. Due to the peak overlap associated with Ba and Ti in EDS,which prevents a quantitative analysis, only SrTi(PPA)₃ was studied indetail by HRTEM/EDS.

To establish the presence of both strontium and titanium in the sameplatelet, HRTEM coupled with EDS was carried out on the SrTi(PPA)₃sample dispersed in toluene. FIG. 3 shows an HRTEM image of a singleplatelet of SrTi(PPA)₃. The metal composition of the indicated area onthe platelet is 51% Sr and 49% Ti. The EDS data collected on severalsuch platelets, summarized in FIG. 16 (images given in SupportingInformation), demonstrate that within experimental error, the ratio ofSr:Ti present in each platelet is 1:1. This is consistent with theresult of XPS analysis of the bulk powder (FIG. 16). Overlap of Ba andTi peaks in EDS prevents quantitative analysis by the this method,however XPS analysis of BaTi(PPA)₃ does show a 1:1 ratio of metals.

Dielectric Properties.

FIG. 9 shows the dielectric constant as a function of inorganic weightloading for polystyrene (PS) composites incorporating the two titanates(BaTiO₃ and SrTiO₃) and the two corresponding mixed metal phosphonates[BaTi(PPA)₃ and SrTi(PPA)₃]. Each data set also includes the value forpure PS, 2.6+0.1, plotted at 0 wt %. For the titanates, the dielectricconstants increase by a factor of nearly four as weight loadingincreases to 40 wt % (to 9.4±0.5 for BaTiO₃, and to 8.6±0.2 for SrTiO₃).The dielectric constants of the mixed metal phosphonates also increaseconsiderably with inorganic weight loading and are significantly higherthan the corresponding titanates at almost every point. At 40 wt %, thedielectric constant of SrTi(PPA)₃ is 66% higher (14.3±0.3) than that ofSrTiO₃. The dielectric constant of BaTi(PPA)₃ is 106% higher (19.4±0.2)than that of BaTiO₃ and more than six times greater than the value forpure PS.

It is also interesting to compare the dielectric constant of a singlemetal phosphonate with that of the mixed metal phosphonate. Thedielectric constant of 40 wt % Ba(PPA) in PS, 4.6±0.1, is significantlylower than the values for either BaTiO₃/PS or BaTi(PPA)₃/PS at the sameweight loading. This suggests that significant enhancement of thedielectric constant depends on the presence of the two different metalsjoined by a “linker” atom in the inorganic phase. Comparing the M—O—Timotif (M=Ba or Sr) in the titanates with the putative M—O—P—O—Ti motifin the phosphonates, the latter apparently leads to a consistentlyhigher increment in the effective dielectric constant of the PScomposite.

IR Spectroscopy

FTIR spectroscopy data (FIG. 9) were collected to gain insight into thebinding of the phenyl phosphonic acid to the mixed metal oxides. Bindingis evidenced through the strong C—H phenyl stretching frequency at 3053cm⁻¹. The C═C aromatic stretches are seen at 1590, 1484, and 1436 cm⁻¹.Two P—O stretches are seem M²⁺—O—P at 1213 cm⁻¹ and M⁴⁺—O—P at 1148cm⁻¹. Finally the out-of-plane bending of the mono-substituted phenylring is seen at 746, 718, and 685 cm⁻¹. The broad absorptions in the2000-2300 cm⁻¹ region is attributed to the coordinated water on thedivalent metal. The coordinated water absorption can be removed byheating sample to 400° C. and is seen in FIG. 10.

Exemplary Mixed Metal Phosphonates (“MMP”)

To date, the present inventors have successfully manufactured mixedmetal phosphonates of the following formulas, where “PPA” representsphenylphosphonic acid as the precursor to the organophosphonate group:ATi(PPA)₃   (Exemplary MMP 1)where A is Mg⁺², Ca⁺², Sr⁺², Ba⁺², Pb⁺², Co⁺², Mn⁺², or Fe⁺²;AZr(PPA)₃   (Exemplary MMP 2)where A is Ba or Pb;LaAl(PPA)₃; and   (Exemplary MMP 3)BaTi(RPO₃)₃   (Exemplary MMP 4)where R is phenyl, octyl, methyl, and carboxyethyl.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood the aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

1. A multi-metal phosphonate of the formula: AB(RPO₃)₃ where A is Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, La³⁺, Co²⁺, Mn²⁺, Fe²⁺, or combinations thereof; B is Ti⁴⁺, Zr⁴⁺, Al³⁺, or combinations thereof; and R is an organic group.
 2. The multi-metal phosphonate as in claim 1, wherein R comprises an aryl group.
 3. The multi-metal phosphonate as in claim 1, wherein R is a phenyl group.
 4. The multi-metal phosphonate as in claim 1, wherein R comprises an alkyl group.
 5. The multi-metal phosphonate as in claim 1, wherein R comprises an alkenyl group.
 6. The multi-metal phosphonate as in claim 1, wherein A comprises Ba²⁺.
 7. The multi-metal phosphonate as in claim 1, wherein B comprises Ti⁴⁺.
 8. The multi-metal phosphonate as in claim 1, wherein the mixed metal phosphonate has the composition: BaTi(PPA)₃, where PPA represents phenylphosphonic acid as the precursor to the organophosphonate group.
 9. The multi-metal phosphonate as in claim 1, wherein the multi-metal phosphonate has the formula: ATi(PPA)₃ where A is Mg⁺², Ca⁺², Sr⁺², Ba⁺², Pb⁺², Co⁺², Mn⁺², or Fe⁺²; and where “PPA” represents phenylphosphonic acid as the precursor to the organophosphonate group.
 10. The multi-metal phosphonate as in claim 1, wherein the multi-metal phosphonate has the formula: AZr(PPA)₃ where A is Ba²⁺or Pb²⁺; and where “PPA” represents phenylphosphonic acid as the precursor to the organophosphonate group.
 11. The multi-metal phosphonate as in claim 1, wherein the multi-metal phosphonate has the formula: LaAl(PPA)₃; where “PPA” represents phenylphosphonic acid as the precursor to the organophosphonate group.
 12. The multi-metal phosphonate as in claim 1, wherein the multi-metal phosphonate has the formula: BaTi(RPO₃)₃ where R is phenyl, octyl, methyl, and carboxyethyl.
 13. A polymeric film comprising a polymeric material and the multi-metal phosphonate of claim
 1. 14. The polymeric film as in claim 13, wherein the polymeric material comprises polystyrene.
 15. The polymeric film as in claim 13, wherein the multi-metal phosphonate is substantially dispersed within the polymeric material. 